Semiconductor vertical cavity devices operate by emitting light in the normal direction to an epitaxial growth surface. In such a device a partial or complete cavity is formed on the same semiconductor substrate that includes the active light emitter. Semiconductor vertical cavity devices include vertical-cavity surface-emitting laser (VCSEL) diodes and resonant cavity light emitting diodes (RCLEDs).
A VCSEL is a laser resonator that includes two minors that are typically distributed Bragg reflector (DBR) mirrors that have layers with interfaces oriented substantially parallel to the die or wafer surface with an active region including of one or more bulk layers, quantum wells, quantum wires, or quantum dots for the laser light generation in between. The planar DBR-mirrors comprise layers with alternating high and low refractive indices. Each layer has a thickness of a quarter of the laser wavelength in the material, yielding intensity reflectivities that may be above 99%.
RCLED's are described in U.S. Pat. No. 5,226,053. A RCLED is a light emitting diode that generates mainly spontaneous emission and generally operates without a distinct threshold. The drive voltage of a spontaneous emitter can be less than its photon energy divided by the electron charge, under which condition it ideally absorbs heat in its light emission process. The RCLED's drive voltage can also exceed its photon energy, under which it generally generates heat in its light emission.
A laser typically generates heat in its light emission process since its quasi-Fermi energy separation for electrons and holes exceeds its emitted photon energy. A laser also generates some heat when electrons and holes absorb some of the laser light but remain respectively in the conduction band or valence band, known as free carrier absorption. In this case, some of the laser light is absorbed, and this internal absorption also reduces the laser light output. A laser also generates some heat if electrical currents are used to transport electrons and holes to an active region with an external electrical bias. These voltage losses that come from the electron and hole charge transport and the optical absorption that comes from the electron and hole free carrier absorption decrease the laser's efficiency and increase its self-heating. The optical absorption increases the laser's threshold which further decreases its efficiency.
Because the light is most efficiently extracted from a vertical cavity diode's surface in the same region in which electrical current is injected into its active area, a particular problem in semiconductor vertical cavity emitting diodes is the efficient lateral confinement of their electrical current to the same region that confines the optical mode. Known approaches to this problem include the standard intra-cavity oxide approach that suffers from mechanical strain, increases the vertical cavity diode's drive voltage, and blocks heat flow inside the vertical cavity diode. What is needed is new lateral electrical confinement structures for vertical cavity semiconductor light sources to confine the optical mode to regions that also receive electrical injection, that can produce high efficiency electrical injection into the same region as the optical mode, and can be fabricated with a high reproducibility across a wafer and from wafer to wafer, that moreover provides minimized mechanical strain and lateral size variation due to external process parameters.